Spent nuclear fuel elements, being radioactive and generating significant thermal energy, require special containers or casks for storage and transportation from a reactor to a reprocessing or storage site.
A number of such spent fuel shipping casks have been designed. Typical construction, as exemplified by U.S. Pat. No. 3,113,215 to Allen, includes a central cavity, an inner container or shell, a radiation shielding filler, an outer container or shell, and heat rejecting fins projecting outwardly from the outer shell.
Federal regulations currently require that a spent fuel shipping cask survive an 802.degree. C. fire for 1/2 hour. The heat rejecting fins, which normally serve to conduct heat away from the cask interior, may then conduct heat inwardly. This, of course, is undesirable as it reduces the ability of a cask to withstand a fire.
One proposed method of fire protection is that of extinguishing the fire by liquids, foams, or gases. If such extinguishing agents are contained within the cask itself, it may be difficult to provide an effective amount of the agent. On the other hand, if the extinguishing agent is located within the carrier used to transport the cask, the extinguishing agent may become ineffective if the cask and carrier become separated.
A method disclosed by U.S. Pat. No. 3,414,727 to Bonilla would cool the cask surrounding the radiation shield with a safety shield of material which melts at a temperature lower than the radiation sheild and adapted to flow out of its enclosed space when subjected to external heat.
Another concept is to incorporate a hydrated substance within the cask. Exposure to an exterior heat source would cause dissociation and vaporization of the contained water; heat would be rejected as latent and sensible heat as the resultant water vapor was vented from the cask. Examples of such substances are: hydrated calcium sulfate (plaster) as disclosed in U.S. Pat. No. 3,466,662 to Blum or hydrated aluminum and iron oxides as disclosed by U.S. Pat. No. 3,780,309 to Bochard. These methods suffer the disadvantage that a limit to the amount of heat that may be rejected is set by the amount of hydrated material that is contained within the cask. Furthermore, if the water within the hydrated substance is relied upon for neutron shielding, that shielding is degraded upon exposure to fire.
A related concept, as disclosed by U.S. Pat. No. 3,737,060 to Blum would place neutron shielding such as borated wood or aluminous cement on the exterior of the outer shell and between the heat rejecting fins. This arrangement presumably would afford some fire protection through charring of the wood or dehydration of the cememt; however it would again suffer the disadvantages noted above.
It is known, especially in the art of designing skins for spacecraft, that certain bimetallic devices may be used for controlling the spacecraft skin temperature by controlling the emissivity of that surface (see, for example, U.S. Pat. Nos. 3,205,937, 3,220,647, 3,307,783, 3,362,467, and 3,411,156). In general, those devices are designed to operate in the cold vacuum of outer space and are effective for radiative heat transfer. The present invention is designed to operate at atmospheric pressure and temperatures ranging from ambient to those found in flames. The present invention is effective in controlling heat transfer by conduction and convection as well as by radiation.
Ablative or intumescent materials could be applied to the surface of the cask, but these materials could interfere with normal fin heat rejection.
Thermal isolation of the cask may be accomplished by irreversibly decoupling the fins from the cask; for example, by melting the fin substructure or by the formation of cavities near the fin surface. These schemes, along with most of the other proposed methods, suffer the disadvantage of irreversibility. Generally, after a fire wanes, human intervention would be required to restore the thermal conductivity of the cask and thereby prevent excessive temperature rise from internal heat generation. Depending on accident conditions, such human intervention may not be possible.